Potassium dichromate
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| Potassium dichromate | |
|---|---|
 |
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| General | |
| Systematic name | Potassium dichromate(VI) |
| Other names | Potassium bichromate |
| Molecular formula | K2Cr2O7 |
| Molar mass | 294.19 g/mol |
| Appearance | Red-orange crystalline solid |
| CAS number | [7778-50-9] |
| EINECS number | 231-906-6 |
| Properties | |
| Density and phase | 2.676 g/cm3, solid |
| Solubility in water | 4.9 g/100 ml (0°C) |
| Melting point | 398°C |
| Boiling point | 500°C decomp. |
| Structure | |
| Coordination geometry |
Tetrahedral (for Cr) |
| Crystal structure | Triclinic (α-form, <241.6 °C) |
| Thermodynamic data | |
| Standard enthalpy of formation ΔfH°solid |
? kJ/mol |
| Standard molar entropy S°solid |
? J.K−1.mol−1 |
| Safety data | |
| Main hazards | Highly toxic Carc. Cat. 2 Muta. Cat. 2 Repr. Cat. 2 Oxidant Dangerous for the environment |
| R/S statement | R: R45, R46, R60, R61, R8, R21, R25, R26, R34, R42/43, R48/23, R50/53 S: S53, S45, S60, S61 |
| PEL-TWA (OSHA) | 0.1 mg/m3 (as CrO3) |
| IDLH (NIOSH) | approx. 15 mg/m3 (as CrVI) |
| Flash point | Non-flammable |
| RTECS number | HX7680000 |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Other anions | Potassium chromate Potassium molybdate Potassium tungstate |
| Other cations | Ammonium dichromate Sodium dichromate |
| Related compounds | Potassium permanganate |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Potassium dichromate, K2Cr2O7 is used in oxidation reactions. As a powerful oxidizing agent, it is the preferred compound for cleaning laboratory glassware of any possible organics.
Potassium dichromate also has important uses in photography and in photographic screen printing. In both cases it is used as an oxidizing agent together with a strong mineral acid.
Chromium intensification uses potassium dichromate together with equal parts of concentrated hydrochloric acid diluted down to approximately 10% v/v to treat weak and thin negatives of black and white photograph roll. This solution reconverts the elemental silver particles in the film to silver chloride. After thorough washing and exposure to actinic light, the film can be redeveloped to its end-point yielding a stronger negative which is able to produce a more satisfactory print.
A potassium dichromate solution in sulfuric acid can be used to produce a reversal negative (i.e,. a positive transparency from a negative film). This is effected by developing a black and white film but allowing the development to proceed more or less to the end point. The development is then stopped by copious washing and the film then treated in the acid dichromate solution. This converts the silver metal to silver sulfate, a compound that is insensitive to light. After thorough washing and exposure to actinic light, the film is developed again allowing the previously unexposed silver halide to be reduced to silver metal.
The results obtained can be unpredictable, but sometimes excellent results are obtained producing images that would otherwise be unobtainable. This process can be coupled with solarisation so that the end product resembles a negative and is suitable for printing in the normal way.
CrVI compounds have the property of tanning animal proteins when exposed to strong light. This quality is used in photographic screen printing. In screen printing a fine screen of bolting silk of similar material has inked squeezed through it onto paper or cloth underneath. If a mask or design is placed on the screen so that ink cannot pass, the design is transferred onto the material underneath. Many artistic prints, T-shirts etc are printed in this manner. To produce a photographic quality print, the screen is first coated with gelatine and then a solution of potassium dichromate is brushed on evenly in dull light. A full size translucent image that is required to be printed is then taped securely onto the surface of the screen and the whole thing exposed to strong light for a period - typically about half an hour in bright sunlight. When the design is removed, the gelatine on the screen is washed off with hot water. All the gelatine exposed to sun-light will have been hardened by the dichromate and will be retained on the screen leaving a precise mask of the required design which can be printed in the usual way.
Potassium dichromate is one of the most common culprits in causing chromium dermatitis. Chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and fore-arms, which is chronic and difficult to treat. As with other CrVI products, potassium dichromate is carcinogenic and should be handled with gloves and appropriate health and safety protection. Potassium dichromate is listed as one of the ingredients in the migraine over-the-counter homeopathic medication called HeadOn, though at a homeopathic dilution of 6X the product contains only one part per million of dichromate.
Dichromate can also drive the oxidation of organic compounds as in 1° Alcohol → Aldehyde → carboxylic acid.
Potassium Dichromate (usually acidified with sulphuric acid), or any other dichromate for that matter, can be used to determine the amount of ethanol in a solution using back titration. First a known amount of dichromate is added, enough to react with all the ethanol and leave some spare in the solution. A blank solution with no ethanol is titrated using sodium thiosulphate with iodide ions (potassium iodide) in the solution and a small amount of stach used as an indicator. The dichromate reacts with the iodide ions forming iodine, which will then react with thiosulphate ions forming iodide ions again. In this way, when all the dichromate has been used up, the thiosulphate will convert iodine to iodide ions and the blue black colour from the starch will disappear, giving a clear endpoint. From here, calculations using balanced formulae easily yield the amount of ethanol in the original solution.Insertformulahere
[edit] Uses in Organic Chemistry
Potassium dichromate (VI) is commonly used as a weak oxidizing agent in the oxidation of alcohols. It converts primary alcohols into aldehydes if distilled immediately upon reaction or carboxylic acids if distilled once the reaction has been taken to completion. Secondary alcohols are converted into ketones, with no further oxidation possible. Tertiary alcohols are not oxidized, since they have no C-H molecular bond to be exploited by the oxidizing agent involved; which can alternatively be sodium dichromate (VI).
